Superconductor mixture

ABSTRACT

A mixture suitable for the production of melt-processed high-temperature superconductors capable of producing a high levitation force. The mixture contains YBa 2  Cu 3  O 7-x  powder with a very low content of copper oxide, i.e. copper not bound in with the YBa 2  Cu 3  O 7-x , and a very low carbon content. Also included are stabilizing (so-called &#34;flux-pinning&#34;) additives. Also disclosed is a method or producing the mixture, as well as YBa 2  Cu 3  O 7-x  powder with suitable low free copper oxide and carbon contents, used to prepare the mixture.

BACKGROUND OF THE INVENTION

The invention relates to a superconductor premixture prepared for fusionprocessing, a method for the production thereof, and a YBa₂ Cu₃ O_(7-x)powder which can be used in preparing the premixture.

YBa₂ Cu₃ O_(7-x) powder (x=0 to 0.5) is used for the preparation oftechnical superconductors. The powder is advantageously mixed withstabilizing additives which form sticking or pinning centers. Theresulting premixture is pressed into compacts and heat treated,preferably by fusion processing. In fusion processing the surfaces ofthe powder particles of the premixture are heat-softened, so that in thecooling that follows the particles are bonded tightly to one another.The flow of the electric current through the grain boundaries is thusimproved. The stabilizing additives usually have the effect of reducingthe melting temperature of the premixture, so that the softening of thebodies that are to formed and the escape of molten material otherwiseobserved at higher fusion texturizing temperatures, can be prevented.The additives, which form sticking or pinning centers, permit increasingthe critical current density in the superconductors. The pinning centersprovide for an anchoring of the magnetic lines of flux, so that highcritical currents can also be achieved in the magnetic field. Fluxpinning additives for superconductors composes of copper oxide aredisclosed, for example, in European Patent Applications EP-A-0 292 126and 296 380.

The bodies obtained by fusion processing from high-temperaturesuperconductor material can be used, for example, in the manufacture ofelectric motors and in the production of the so-called "flywheel." Theflywheel rotates under superconducting conditions in the magnetic fieldand serves to store kinetic energy.

SUMMARY OF THE INVENTION

It is the object of the present invention to provide an improvedpremixture composed of YBa₂ Cu₃ O_(7-x) powders with stabilizingadditives or those producing flux pinning. This object is achieved bythe premixture described below.

The invention is based on the knowledge that premixtures which comprisethe YBa₂ Cu₃ O_(7-x) powders and stabilizing or flux pinning additivesare especially well suited for the preparation of fusion-processedbodies if they have a small content of carbon combined with a smallcontent of free copper oxide not bound in the YBa₂ Cu₃ O_(7-x) phase.

The superconductor premixture according to the invention, which isdesigned for fusion processing, comprises additives which produce fluxpinning as well as YBa₂ Cu₃ O_(7-x) powder, this YBa₂ Cu₃ O_(7-x) powdercontaining less than 0.6 wt.-% of free copper oxide not bound in theYBa₂ Cu₃ O_(7-x) phase and less than 0.1 wt.-% carbon, relative to theYBa₂ Cu₃ O_(7-x) powder.

The stabilizing or flux pinning additives can basically be added in anyamount in which they result in the desired effect. The stabilizingaction lies, for example, in the fact that the melting temperature ofthe particles in the superconductor premixture is lowered. The additivesthat produce the flux pinning cause the magnetic flux lines to beanchored in the superconducting material, and consequently high criticalcurrents become possible. Any customary stabilizing or flux pinningadditives can be used. There are additives, such as Y₂ BaCuO₅, of whichit is assumed that they have both a stabilizing and a flux pinningaction. Especially well suited as additives are Y₂ O₃, Y₂ BaCuO₅, PtO₂,Ag₂ O, CeO₂, SnO₂, ZrO₂, BaCeO₃ and BaTiO₃.

Preferred superconductor premixtures according to the invention contain0.1 to 50 wt.-% of stabilizing additives or additives that produce fluxpinning, relative to the YBa₂ Cu₃ O_(7-x) powder taken as 100%.

A content of Y₂ O₃, Y₂ BaCuO₅, PtO₂, Ag₂ O, CeO₂, SnO₂, ZrO₂, BaCeO₃and/or BaTiO₃ is preferred as stabilizing or flux pinning additives inthe superconductor premixture according to the invention.

If Y₂ O₃ is contained, an addition of 0.1 to 50 wt.-% of this compound,with respect to the YBa₂ Cu₃ O_(7-x) powder taken as 100 wt.-%, is veryadvantageous; if Y₂ BaCuO₅ is contained, 0.1 to 50 wt.-% isadvantageous. If PtO₂ is contained, 0.5 to 5 wt.-% of PtO₂ additive isadvantageous. If Ag₂ O is contained, and an addition of 1 to 20 wt.-% isadvantageous. If CeO₂, SnO₂, ZrO₂, BaCeO₃ and/or BaTiO₃ is contained, acontent of 0.1 to 5 wt.-% is preferred.

A superconductor premixture of the invention with especiallyadvantageous properties in fusion texturizing contains less than 0.05wt.-% of carbon, relative to the YBa₂ Cu₃ O_(7-x) phase. Free copperoxide not bound in the YBa₂ Cu₃ O_(7-x) phase is preferably contained inan amount of less than 0.2 wt.-%.

It has proven to be advantageous if the superconductor premixturecomprises particles in the grain size range from above 30 up to 500 μm.It is especially advantageous if 40 to 70 wt.-% of the amount is in thestated grain size range.

If desired, the superconductor premixture according to the invention canbe in compressed form, for example in the form of compacts pressedaxially or isostatically.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The preparation of the superconductor premixture of the invention willbe described below. For this purpose a YBa₂ Cu₃ O_(7-x) powdercontaining less than 0.6 wt.-% of free copper oxide not bound in theYBa₂ Cu₃ O_(7-x) phase, relative to the YBa₂ Cu₃ O_(7-x) powder, andless than 0.1 wt.-% of carbon relative to the YBa₂ Cu₃ O_(7-x) powder,is mixed according to the invention with stabilizing and/or flux pinningadditives. The mixture, i.e., the premixture, can then be pressed ifdesired, axially or isostatically for example, to form compacts. Powderwith preferred properties, the especially good stabilizing and/or fluxpinning additives, and their preferred amounts, have already beenmentioned above.

The preparation of a YBa₂ Cu₃ O_(7-x) powder is explained hereinafter,which has the specification required for use in the superconductorpremixture of the invention with regard to the free copper oxide contentand the carbon content. This powder, which is especially suitable foruse in the superconducting premixture of the invention is likewisesubject matter of the invention.

This YBa₂ Cu₃ O_(7-x) powder according to the invention, usable tospecial advantage in the superconductor premixture, is prepared bymixing together powdered yttrium, barium and copper compounds whichinclude anions which have oxygen as well as, in some cases, hydrogenand/or carbon, and which are used in such amounts that the atomic ratioof yttrium, barium and copper is substantially 1:2:3,

a) calcining the finely divided starting material by heating it to atemperature of 850 to 920° C. in an oxygen-containing atmosphere,holding it at this temperature, then cooling it to room temperature, andthen finely dividing the partially sintered powder body,

b) repeating step a) one or more times, and after the last repetition,upon cooling in the temperature range of 380 to 420° C., inserting aholding phase of up to 50 hours, until the mixture obtained is fullyoxidized, and in the obtained mixture the detectable residue of copperoxide not bound in the YBa₂ Cu₃ O_(7-x) phase is less than 0.6 wt.-% andcarbon less than 0.1 wt.-% with respect to the YBa₂ Cu₃ O_(7-x) powder,and

c) comminuting the material obtained in step b).

It is advantageous to use a powder in which at least 40% of the materialis in the grain size range of 30 to 500 μm. It is especially preferredto prepare the superconductor premixture of the invention by admixing aYBa₂ Cu₃ O_(7-x) powder in which 40 to 70 wt.-% of the material is inthe grain size range of 30 to 500 μm. If the powder is produced by themethod described above, it is preferred to grind, and optionally sort,the material after the holding phase.

The YBa₂ Cu₃ O_(7-x) powder of the invention is advantageously preparedby using Y₂ O₃, CuO and barium hydroxide, hydrated barium hydroxide orbarium carbonate.

The calcination in the above-described step a) can advantageously beperformed by calcining the starting material in loose or precompressedform by initially heating it in air at a rate of 100 to 400° C./hourfrom ambient temperature to an end temperature of 850 to 920° C.,letting it stand at this temperature for 100 to 400 hours and thencooling it to ambient temperature at 100 to 200° C. per hour. Theresulting material can then be ground as mentioned above, if desired,and sorted, if desired. At the end of the holding phase, the YBa₂ Cu₃O_(7-x) powder is advantageously mixed with the additive or additivesand, if desired, axially or isostatically pressed to form compacts.

The superconductor premixture according to the invention isoutstandingly suitable for the preparation of fusion-processedhigh-temperature superconductors with high levitation power. Thecompacts which are made with the premixture according to the inventionare very stable; in tests of large numbers of samples they exhibitreproducible high levitation forces.

The following example is intended to further explain the inventionwithout limiting its scope.

EXAMPLE

a) Preparation of the YBa₂ Cu₃ O_(7-x) powder.

The yttrium compound used was commercial Y₂ O₃ with a purity of 99.99%,the copper compound was commercial CuO with a purity of 99.999% and thebarium compound commercial BaCO₃ with a purity of 99.4%. The startingmaterials were measured out according to the desired composition forproducing an amount of 500 g, and were homogeneously mixed. The mixturewas loosely filled into in Kawenite crucibles (diameter 50 mm, height 35mm). The crucibles were placed in a laboratory muffle kiln with lowtemperature gradients (ΔT<1 K/cm) and heated at 300 K/hour to an endtemperature of 905° C. The powder was held at this temperature for 96hours and thereafter cooled at 200 K/hour to room temperature. Thepowder cake was crumbled and sieved, and the crucibles were again filledwith this granulate. The granulate was subjected to a second heatingcycle over 72 hours, to a third heating cycle over 62 hours, and to afourth heating cycle over 70 hours, at the same temperature and at equalheating and cooling rates, and crumbled and sieved after each heatingcycle. After the last cycle the granulate was held at 400° C. for onehour, cooled at 200 K/hour to room temperature, and comminuted. All ofthe heating cycles were performed in air. The detectable free copperoxide in the fully reacted powder was measured by differential thermalanalysis and thermogravimetry, and is under 0.2 wt.-%. The carboncontent according to chemical analysis is less than 0.05 wt.-%. Theextensive conversion of the premixture to the orthorhombic phase wasdetectable by means of the doublet cleavage in the X-ray diffractionspectrum.

b) Verification of the Premixture.

The powder was premixed with 9 wt.-% of yttrium oxide and 1 wt.-% ofplatinum oxide. The premixture was pressed axially at 3 MPa to compacts35 mm in diameter and 20 mm in height. The compacts were heated at 300K/hour to 1190° C. and held for 0.5 hours, then cooled at 400 K/hour to980° C., and slowly fusion-processed from this temperature at 1 K/hourto 950° C. in the temperature gradient of 5 K/cm. Cooling to roomtemperature was performed at 400 K/h. In a separate heat process between400 to 600° C. over 144 hours the material was converted to theorthorhombic phase. In the superconductive state at 77 Kelvin, when thecompact approaches a samarium-cobalt permanent magnet 25 mm in diameterand 10 mm in thickness, at a distance of 0.5 mm levitation forces of 40to 60 N are produced, corresponding to 30 to 35% of the repulsiveforces, for perfect shielding.

What is claimed is:
 1. Powdered YBa₂ Cu₃ O_(7-x) containing greater thanzero percent to less than 0.6 wt.-% of free copper oxide not bound inthe YBa₂ Cu₃ O_(7-x) phase and less than 0.1 wt.-% of carbon, said wt.-%values being calculated relative to the weight of YBa₂ Cu₃ O_(7-x).
 2. Asuperconductor premixture prepared for fusion processing, comprisingYBa₂ Cu₃ O_(7-x) powder and at least one stabilizing additive or fluxpinning additive, wherein said YBa₂ Cu₃ O_(7-x) powder contains greaterthan zero percent to less than 0.6 wt.-% of free copper oxide not boundin the YBa₂ Cu₃ O_(7-x) phase and less than 0.1 wt.-% of carbon,relative to the weight of the YBa₂ Cu₃ O_(7-x) powder.
 3. Asuperconductor premixture according to claim 2, wherein said premixturecomprises from 0.1 to 50 wt.-% of said stabilizing additive or fluxpinning additive, relative to the weight of the YBa₂ Cu₃ O_(7-x) powder.4. A superconductor premixture according to claim 2, wherein said YBa₂Cu₃ O_(7-x) powder contains greater than zero to less than 0.2 wt.-% offree copper oxide not bound in the YBa₂ Cu₃ O_(7-x) phase.
 5. Asuperconductor premixture according to claim 2, wherein said YBa₂ Cu₃O_(7-x) powder contains less than 0.05 wt.-% of carbon with respect tothe YBa₂ Cu₃ O_(7-x) phase.
 6. A superconductor premixture according toclaim 2, wherein at least 40% of the YBa₂ Cu₃ O_(7-x) powder iscomprised of powder particles having a grain size in the range from 30to 500 μm.
 7. A superconductor premixture according to claim 2, whereinsaid stabilizing additive or flux pinning additive comprises yttriumoxide, Y₂ O₃, platinum dioxide, Ag₂ O, CeO₂, SnO₂, BaCeO₃, BaTiO₃ andmixtures of two or more thereof.
 8. A superconductor premixtureaccording to claim 7, wherein said stabilizing additive or flux pinningadditive comprises at least one substance selected from the groupconsisting of 0.1 to 50 wt.-% of Y₂ O₃, 0.1 to 50 wt.-% of Y₂ BaCuO₅,0.5 to 5 wt.-% of PtO₂, 1 to 20 wt.-% Ag₂ O and 0.1 to 5 wt.-% of atleast one of CeO₂, SnO₂, ZrO₂, BaCeO₃ and BaTiO₃, said wt.-% valuesbeing calculated relative to the weight of the YBa₂ Cu₃ O_(7-x).
 9. Amethod of preparing a superconductor premixture for fusion processing,said method comprising admixing YBa₂ Cu₃ O_(7-x) powder containinggreater than zero percent to less than 0.6 wt.-%, relative to the YBa₂Cu₃ O_(7-x), of free copper oxide not bound in the YBa₂ Cu₃ O_(7-x)phase and less than 0.1%, relative to the YBa₂ Cu₃ O_(7-x), of carbonwith at least one stabilizing additive or flux pinning additive.
 10. Amethod according to claim 9, further comprising the step of pressing themixture axially to a compact.
 11. A method according to claim 9, furthercomprising the step of pressing the mixture isostatically to a compact.12. A method according to claim 9, further comprising preparing saidYBa₂ Cu₃ O_(7-x) by:providing a mixture of finely divided yttrium,barium and copper starting compounds which include oxygen-containinganions, said mixture containing said yttrium, barium and coppercompounds in amounts such that yttrium, barium and copper are present inthe mixture in an atomic ratio of substantially 1:2:3; calcining themixture by heating to a temperature of 850 to 920° C. in anoxygen-containing atmosphere, holding the mixture at that temperature,and then cooling the mixture to room temperature, whereby a partiallysintered product is produced; comminuting the partially sinteredproduct; repeating the foregoing calcining and comminuting steps atleast one, and during the cooling of the last repetition, holding in atemperature range of 380 to 420° C. for a holding phase of up to 50hours until the mixture is completely oxidized and the amount of copperoxide detectable in the mixture, but not bound in the YBa₂ Cu₃ O_(7-x)phase, is greater than zero percent to below 0.6 wt.-% and the amount ofcarbon is less than 0.1 wt.-%, said wt.-% values being calculatedrelative to the weight of YBa₂ Cu₃ O_(7-x).
 13. A method according toclaim 12, wherein said anions contain at least one additional elementselected from hydrogen and carbon.
 14. A method according to claim 12,wherein after the comminution of the last repetition, at least 40 wt.-%of the comminuted material has a grain size in the range of 30 to 500μm.
 15. A method according to claim 12, wherein after the comminution ofthe last repetition, the comminuted mixture is sorted to collect aproduct in which at least 40 wt.-% has a grain size in the range of 30to 500 μ.
 16. A method according to claim 12, wherein the comminutedproduct from the last repetition is admixed with the at least oneadditive, and further comprising pressing the mixture to a compact. 17.A method according to claim 12, wherein the yttrium starting compound inY₂ O₃, the starting copper compound is CuO, and the starting bariumcompound is selected from barium hydroxide, hydrated barium hydroxideand barium carbonate.
 18. A method according to claim 12, wherein themixture of starting compounds is calcined in loose or slightlycompressed form by initially heating it in air at a rate of 100 to 400°C./hour from ambient temperature to an end temperature of 850 to 920°C., holding at this temperature for 100 to 400 hours, and then coolingto ambient temperature at 100 to 200° C. per hour.
 19. A methodaccording to claim 12, wherein the product is ground and sorted so that40 to 70 wt.-% thereof has a grain size in the range of 30 to 500μm.